This invention relates to the field of downhole measurement-while-drilling methodologies and especially to the reliability of those methodologies.
To obtain hydrocarbons such as oil and gas, boreholes are drilled by rotating a drill-bit attached at a drill string end. A large proportion of the current drilling activity involves directional drilling, i.e., drilling deviated and horizontal boreholes to increase the hydrocarbon production and/or to withdraw additional hydrocarbons from the earth's formations. Modern directional drilling systems generally employ a drill pipe having a drill bit at the bottom that is rotated by a drill motor or a “mud motor”. Pressurized drilling fluid or “mud” or “drilling mud” is pumped into the drill pipe to rotate the drill motor and to provide lubrication to various members of the drill string including the drill bit. The drill bit and drill motor form part of what is known as the bottom hole assembly (“BHA”). As required the drill pipe is rotated by a prime mover, such as a motor, to facilitate directional drilling and to drill vertical boreholes.
Measurement-While-Drilling (MWD) surveying for directional and horizontal drilling processes is performed to provide the orientation and the position of the BHA. State of the art MWD surveying techniques are based on magnetic surveying which incorporates three-axis magnetometers and three-axis accelerometers arranged in three-mutually orthogonal directions. The three-axis accelerometers monitor the Earth gravity field to provide the inclination and the tool face angles. This information is combined with the magnetometer measurements of the Earth magnetic field to provide the azimuth.
Measurement-While-Drilling tools were developed in the 1970's, and by the 1980's MWD became a key component of the industries growing trend to drill directional wells.
MWD tools allow near bit downhole measurements to be made while the well is being drilled and data is transmitted to the surface using a mud pulse telemetry. These “real time” directional and formation evaluation measurements are significant and instrumental to the drilling of useful and cost effective boreholes. During the 1990's MWD technology become a requirement and integral part of the drilling of exploration and directional wells.
Measurement-while-drilling tools are complex assemblies of high-tech electronics. These tools use accelerometers and magnetometers to measure the inclination and azimuth of the wellbore at that location and are able to transmit that information to the surface. They can also provide information about the conditions at the drill bit. This may include:
Rotational speed of the drill string
Smoothness of that rotation
Type and severity of any vibration downhole
Downhole temperatures
Torque and weight on the drill bit
Mud flow volume
Many MWD tools, either on their own, or in conjunction with separate Logging While Drilling tools, can take measurements of formation properties. At the surface, these measurements are assembled into a log, similar to one obtained by wireline logging. LWD tools are able to measure a suite of geological characteristics including—density, porosity, resistivity, pseudo-caliper, inclination at the drill bit, magnetic resonance and formation pressure. The MWD tool allows these measurements to be taken and evaluated while the well is being drilled. This makes it possible to perform geo-steering, or directional drilling based on measured formation properties, rather than simply drilling into a preset target. Most MWD tools also contain an internal Gamma Ray sensor to measure natural Gamma Ray values. This is because these sensors are compact, inexpensive, reliable, and can take measurements through unmodified drill collars. Other measurements often require separate Logging While Drilling tools, which communicate with the MWD tools downhole through internal wires.
The most common method of data transmission used to communicate all of this information back to the surface is mud pulse telemetry. Continuous wave telemetry sends the sensory data accumulated by the downhole measurement tool, known as an MWD (measurement while drilling) or LWD (logging while drilling) tool, by phase variations in the rig standpipe in a specific analog signal sequence that can be converted to a digital signal by processors that can measure fluctuations in the medium's pressure. Positive pulse telemetry sends the sensory data by pressure increases. Negative pulse is the same transmission of encoded data using pressure decreases.
MWD tools are placed in the drilling assembly near to the drilling bit and are exposed to the high temperatures, pressures, shocks and vibrations associated with the drilling process. This harsh operational environment is not conducive to the long-term health of the MWD System. As the industry become more reliant on MWD tools the reliability of these tools became increasing important. A metric of reliability often to used gauge MWD tools over a significantly statistical operation history is “Mean Time Between Failures” (MTBF).
Today MWD tools are “mission critical” and an MWD tool failure requires the rig operator to replace the tool by pulling the drill pipe out of the well. MWD MTBF is now a significant economic factor in the drilling of a borehole. MWD tool manufactures have been mandated to increase the functionality of the tools while also increasing the tools MTBF.
Failed MWD tools are often run back into the borehole with the same failure problem as experienced during the first failed run. Repair technicians and tool operators often have to use intuition to replace suspect parts and/or change downhole-running conditions to attempt to “fix” the tool.
MWD tool operators attempting to increase MWD MTBF must either (1) know the root cause of failure for the tool, or (2) repair the system by either changing out a defective tool component or changing the operational parameters that the tool is run under.
MWD systems can fail in many ways some typical causes of failure are:
a. Hard system or component failure.
b. Temperature related component failure.
c. Intermittent connections due to temperate and/or vibration effects.
d. MWD pulser wear, erosion or sticking due to drilling fluid properties.
Of the failure modes above only a) “hard systems or component failure” can be demonstrated at the surface in a shop environment. Failure modes b) thru d), cannot be definitely reproduced or understood in the shop environment and confidently repaired.
There is an important need then in this industry for a method and apparatus that will be able to record b) thru d) types of failures so these failures can be confidently identified and resolved. This will result in a more reliable tool string that in turn will result in an increase in mean-time-between-failures.